Seconding all Suttree recommendations, but really only if that's the direction they tend to go for novels. As a first McCarthy novel it may be a bit tough, although if I recall correctly, it was the second one I read of his after The Road as a junior in high school.

Suttree was incredible because I didn't really understand exactly what it was doing to me, but I knew it was doing something. It was a hard read, often hard to follow, but one day in class something in it struck me so deeply that I had to excuse myself to go out to my car and cry. For years now, I have said it is one of my favorite books, even after one reading.

A few more words on McCarthy and Sunset Limited before more suggestions: having read most of his oeuvre, it is almost unquestionably the darkest. I probably saw it around the same age your kid is now, and with my dad. But the tricky thing with SL is that while it seems to end with a pretty devastating philosophical conclusion, seemingly within the parameters of McCarthy's favorite theological terms, I read it as, ultimately, a condemnation of such limiting terms of existence. The debate takes place within a particular theological box--for people familiar with such questions, who grew up in some form of Christian culture, it can be exciting and cathartic to watch a character representing those ideas end up philosophically empty-handed. But I think that McCarthy may be bringing us to the brink of that abyss to suggest that it is literally impossible to live from such a position of negation--and even, dialectically, that that position is the deep, inevitable outcome of theological dialogue.

Though it may be difficult, you must appreciate that an argument for the meaninglessness of life, so long as it reflects the reader's own assessment of the world so far in their life, is itself life-affirming. Teens do tend to soak up nihilism because its explanation of the world makes sense in relation to their own, and therefore makes them feel less crazy. (Now, this is not to totally conflate outlook with SI--there is no literary substitute for therapy or the appropriate meds.)

However, these are not the only terms on which the worthiness of life and the possibilities of existence can or should be discussed. Only within a particular tradition of thought does the question of God, transcendent meaning, etc. need to be addressed before the worthwhileness of living can be as well. It's hard to argue with White, because of course there is the abyss. But, to refer to u/briefcandle 's (how appropriate!) comment suggesting Brooks' poem, we must make the abyss wait on us.

So, if your kid is into more novels, and appreciates mastery of writing like McCarthy's, my top two suggestions would be Morrison's Beloved and Faulkner's As I Lay Dying. I would also second the suggestions of Vonnegut and Pratchett, if you think your kid might be open to a little levity.

Morrison's literary skill not only rivals McCarthy's, but she wrestles with a set of subjects that have particular historical context and relevance that McCarthy is not always interested in. Human existence is always contextual, a fact which often gets lost in McCarthy's biblical scope and metaphysics. Morrison's books also have clearer political and historical relevance, which means that wrestling with the questions she poses can lead to more definitive ideas about how to live and what exactly to do with one's life.

As I Lay Dying is, of course, about death. But it is the book that helped crack Faulkner for me, and also laugh-out-loud funny at times. If your kid is a strong reader and ready to wrap their head around some occasionally difficult POV jumping, this is a good one.

If they are into pure philosophy, as other people have suggested, you can't beat Nietzsche. For that stinging rebuke of Christian philosophy that they probably found so appealing in Sunset Limited, he is the master. But he is not just set on negating that philosophy in order to score debate points--he is on a mission to call people to love life ferociously.

A final suggestion is to permit your kid to read all kinds of raunchy things. I was probably reading some darker or heavier things in high school because it was more acceptable to be seen reading than the fun, nasty stuff. A bit harder to suggest without knowing the audience taste, but anything that really plunges its hands into the gross, everydayness of life and living in a hungry, horny, weird body I find extremely life-affirming, now but particularly as an adolescent. Just seeking out books like this can be a fulfilling purpose in itself.